Volume Control unresponsive


I am playing, from tidal, to a Naim Mu-so (AirPlay :scream:) and had control from my iPad. Shortly after, from a different location, I tried to reduce volume using my iPhone which didn’t work. After that there was no control of volume from either device, even after restart of apps & Mu-so!
I finally managed to regain control after transferring output to another device/zone and back. This is not the first time this has happened but the bug seems inconsistent.
In an ideal situation I guess I should use the same device for control, alas that’s not always possible.
Is this a known problem/Any thoughts?